Invasive Shrubs to Avoid

Fall is a great time to plant trees and shrubs. The soil is still warm, and rainfall is usually sufficient enough to establish roots before the ground freezes. Plant well before the first frost so plants can stabilize themselves before the soil temperature drops below 42°F. A layer of mulch will help minimize rapid fluctuations in soil temperature and maintain soil moisture. Choose balled-and-burlapped plants or those growing in containers for best results.

When searching for plants to add to your garden, avoid anything on the Pennsylvania invasive plant list, even if they have lovely flowers, make a perfect color contrast, or are at a bargain price. Even though considered invasive, many of these plants will be available at local nurseries and you may already have them growing on your property.

Invasive plants spread quickly because they are not limited by pests, diseases, or predators. They grow and reproduce rapidly and are able to thrive in a variety of situations, even adverse conditions. They often spread by rhizomes and produce numerous seeds that will germinate quickly in disturbed ground.

Four particularly common and eye-catching invasive plants are burning bush (Euonymus alatus) with its brilliant red fall color, privet (Ligustrum obtusifolium) frequently used as a hedge, Japanese barberry (Berberis thunbergii) and butterfly bush (Buddleja davidii) planted to attract butterflies.

Burning bush

Or winged euonymus, has bright red fall foliage and has been used as an ornamental shrub in the United States since the 1860's. It is commonly used in home and commercial landscapes as a foundation plant or as a hedge. A native of northeastern Asia, it is hardy to USDA zone 4 and frequently escapes cultivation into woodlands, forests and fields. Since it tolerates full shade and a variety of soil types, it out competes native plants and creates large thickets, crowding out native species and shading others. This fast-growing species has grayish brown stems with corky wings.  Leaves are small, opposite, and finely serrated along the margins. Flowers are inconspicuous but the red fruit is eaten and spread by birds causing rapid infestations. Small seedlings can be hand-pulled while larger ones must be dug out or cut down.

This photo of burning bush on the forest floor illustrates how invasive plants can replace native vegetation and create a monoculture.  Photo credit: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Privet

Several different species of privets, used in landscaping and as hedges, have become naturalized in Pennsylvania but the most common is obtuse-leaved privet. It is a deciduous shrub with multiple stems and light gray twiggy branches. The opposite leaves are oval tapering slightly at the tip and base. They appear dark green in summer and become purplish in fall. Obtuse-leaved privet blooms in June and produces small white flowers that occur in clusters at the end of branches. Black fruits mature in September and are distributed by birds to stream valleys, fields, fence rows, and rights-of-way.

When cut back severely, privet can regenerate from roots making it difficult to control. When it forms dense thickets, light and moisture are less available to native plants and wildflowers reducing diversity. Privet is difficult to remove once established but can be pulled by hand when young. Cutting is effective on larger plants but the process will need to be repeated.

Japanese barberry

This shrub is widely used as a foundation plant, hedge, and in mixed borders.  It was brought to the United States from Japan in 1864 and is currently one of the most widely invasive non-native plants in Pennsylvania. This spiny shrub has oval, serrated leaves that are often tinged with red. The twigs have a single sharp spine at each node and the yellow flowers appear on the underside of branches in April or May. Bright red berries are eaten by birds and small mammals who disperse the seeds.

Since deer do not eat barberry, it has become a dominant plant in woodlands, forming thick underbrush that displaces native plants and reduces wildlife habitat. It is also considered a human health hazard because it can act as a nursery for black-legged ticks that have the potential to spread Lyme disease.

Wearing thick gloves, pull small plants by hand. Dig up larger plants making sure to remove the entire root system and dispose of all plant parts.

Butterfly bush

This shrub appears to be the ideal plant for a pollinator garden but like other non-natives it has no natural predators and not one native caterpillar feeds on the leaves. This allows the plant to spread and displace native shrubs that could act as a host plants for butterflies. Lance-shaped leaves are grey-green and opposite on arching stems. Purple, pink, and white flowers bloom on long spikes from May to August. Seeds are abundant and easily dispersed by wind to roadsides, hedgerows, fields, train tracks and forest edges. Butterfly bush prefers well-drained soil but can survive in dry conditions once established. Seedlings can be pulled out or dug.

Use Native plants

Consider the following native shrubs for your fall planting: red chokeberry (Aronia arbutifolia), witch-hazel (Hamamelis virginiana), winterberry holly (Ilex verticillata), ninebark (Physocarpus opulifolius), Virginia sweetspire (Itea virginica), inkberry holly (Ilex glabra), wild hydrangea (Hydrangea arborescens), bayberry (Myrica pensylvanica), silky dogwood (Cornus racemosa) and New Jersey tea (Ceanothus americanus).

Native shrubs that attract butterflies include sweet pepperbush (Clethra alnifolia) and Virginia sweetspire (Itea virginica).